Mineralogy and mineral chemistry of detrital heavy minerals from the Rhine River in Germany as evidence to their provenance, sedimentary and depositional history: focus on platinum‑group minerals and remarks on cassiterite, columbite‑group minerals and uraninite

Abstract

In the course of studying the gold-bearing
heavy mineral spectrum of sediments from the upper Rhine
River, a distinct suite of detrital grains comprising platinum-
group minerals (PGM), cassiterite, columbite-group
minerals and uraninite was identified and investigated
using conventional and modern analytical methods. This
study aimed to characterize the selected mineral groups
mineralogically and geochemically in order to identify possible
source areas and to reconstruct different aspects of the
complex sedimentary history of the Rhine River sediments.
The PGM assemblage is dominated by grains of Ru–Os–
Ir alloys (~70 %), followed by Pt–Fe alloys, sperrylite and
rare other PGM. Accordingly, this PGM assemblage represents
highly mature, physically and chemically extremely
resistant compounds which may have experienced and
survived repeated reworking during their sedimentary history.
Pt–Fe alloys and sperrylite may originate from various
sources; however, the predominant Ru–Os–Ir alloy grains
point to an origin from ophiolite sequences of unknown age
(but likely pre-Alpine; Variscan or older). The exact locations
of the primary sources and the complex, prolonged
sedimentary history of the detrital PGM with possibly multiple
intermittent storages remain unknown. Detrital cassiterite
grains were dated by the U–Pb method using LAICP-
MS. The age dates of cassiterite largely overlap with
zircon age distributions by peaking distinctly at ca. 325 Ma
(majority of ages), thereby implying a predominantly Variscan
age of the cassiterite grains and possible derivation
from mineralization in the Black Forest area. Columbitegroup
minerals are dominantly tapiolite originating from
pegmatites. Rare uraninite grains attest that this mineral
experienced rapid erosion, transport and deposition in a
reducing environment.